Very High Energy Emission and Cascade Radiation of Gamma-Ray Burst Afterglows: Homogeneous Versus Wind External Media

TL;DR

This paper compares the high-energy emission and cascade radiation of gamma-ray burst afterglows in homogeneous versus wind external media, highlighting differences in flux, light curves, and cascade effects.

Contribution

It provides a detailed analysis of SSC and cascade emissions in different environments, incorporating absorption effects and Klein-Nishina suppression, which is novel in GRB afterglow modeling.

Findings

Higher SSC flux in denser environments due to larger Compton parameter.

Faster decrease of SSC flux ratio in wind medium over time.

Cascade emission can significantly affect early optical and GeV afterglow light curves.

Abstract

Recent detection of sub-TeV emission from gamma-ray bursts (GRBs) represents a breakthrough in the GRB study. The multi-wavelength data of the afterglows of GRB 190114C support the synchrotron self-Compton (SSC) origin for its sub-TeV emission. We present a comparative analysis on the SSC emission of GRB afterglows in the homogeneous and wind environment in the framework of the forward shock model. The $\gamma\gamma$ absorption of very high-energy photons due to pair production within the source and the Klein-Nishina effect on the inverse-Compton scattering are considered. Generally a higher SSC flux is expected for a larger circum-burst density due to a larger Compton parameter, but meanwhile the internal $\gamma\gamma$ absorption is more severer for sub-TeV emission. The flux ratio between the SSC component and the synchrotron component decreases more quickly with time in the wind medium case than that in the homogenous-density medium case. The light curves of the SSC emission are also different for the two types of media. We also calculate the cascade emission resulted from the absorbed high-energy photons. In the ISM environment with $n> 1\,\rm cm^{-3}$, the cascade synchrotron emission could be comparable to the synchrotron emission of the primary electrons in the optical band, which may flatten the optical afterglow light curve at early time ($t<1$ h). In the wind medium with $A_{\ast}> 0.1$, the cascade emission in the eV-GeV band is comparable or even larger than the emission of the primary electrons at early time.